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The Native File System API: Simplifying access to local files

The Native File System API (formerly known as the Writeable Files API), is
available behind a flag in Chrome 77 and later, and should begin an origin
trial in Chrome 78 (stable in October). It is part of our capabilities
project, and this post will be updated as the implementation progresses.

What is the Native File System API?

The new Native File System API enables developers to build powerful web apps
that interact with files on the user's local device, like IDEs, photo and video
editors, text editors, and more. After a user grants a web app access, this
API allows web apps to read or save changes directly to files and folders
on the user's device.

We've put a lot of thought into the design and implementation of the Native
File System API to ensure that people can easily manage their files. See the
Security and permissions
section of this post.

Using the Native File System API

To show off the true power and usefulness of the Native File System APIs,
I wrote a single file text editor. It lets you open a text
file, edit it, save the changes back to disk, or start a new file and save
the changes to disk. It's nothing fancy, but provides enough to help you
understand the concepts.

Enabling via chrome://flags

If you want to experiment with the Native File System API locally, enable
the #native-file-system-api flag in chrome://flags.

Read a file from the local file system

The first use case I wanted to tackle was to ask the user to choose a file,
then open and read that file from disk.

Ask the user to pick a file to read

The entry point to the Native File System API is
window.chooseFileSystemEntries(). When called, it shows
a file picker dialog, and prompts the user to select a file. After selecting
a file, the API returns a handle to the file. An optional options parameter
lets you influence the behavior of the file picker, for example, allowing the
user to select multiple files, or directories, or different file types.
Without any options specified, the file picker allows the user to select a
single file, perfect for our text editor.

Like many other powerful APIs, calling chooseFileSystemEntries() must be
done in a secure context, and must be called from within
a user gesture.

Once the user selects a file, chooseFileSystemEntries() returns a handle,
in this case a FileSystemFileHandle that contains the
properties and methods needed to interact with the file.

It’s helpful to keep a reference to the file handle around so that it
can be used later. It’ll be needed to save changes back to the file, or
to perform any other file operations. In the next few milestones, installed
Progressive Web Apps will also be able to save the handle to IndexedDB
and persist access to the file across page reloads.

Read a file from the file system

Now that you have a handle to a file, you can get the properties of the file,
or access the file itself. For now, let’s simply read the contents of the
file. Calling handle.getFile() returns a File object,
which contains binary data as a blob. To get the data from the blob, call
one of the reader methods (slice(), stream(), text(), arrayBuffer()).

Write the file to the local file system

In the text editor, there are two ways to save a file: Save, and Save As.
Save simply writes the changes back to the original file using the file
handle we got earlier. But Save As creates a new file, and thus requires a
new file handle.

Create a new file

The chooseFileSystemEntries() API with {type: 'saveFile'} will show the
file picker in “save” mode, allowing the user to pick a new file they want
to use for saving. For the text editor, I also wanted it to automatically
add a .txt extension, so I provided some additional parameters.

Save changes to the original file

To write data to disk, I needed to create a FileSystemWriter
by calling createWriter() on the file handle, then call write() to do
the write. If permission to write hasn’t been granted already, the browser
will prompt the user for permission to write to the file first (during
createWriter()). The write() method takes a string, which is what we
want for a text editor, but it can also take a BufferSource,
or a Blob.

Note: There's no guarantee that the contents are written to disk until
the close() method is called.

The keepExistingData option, when calling createWriter(), isn’t supported
yet, so once I get the writer, I immediately call truncate(0) to ensure I
start with an empty file. Otherwise, if the length of the new content is
shorter than the existing content, the existing content after the new
content would remain. keepExistingData will be added in a future milestone.

When createWriter() is called, Chrome checks if the user has granted
write permission. If not, it requests permission. If the user grants
permission, the app can write the contents to the file. But, if the user
does not grant permission, createWriter() will throw a DOMException, and
the app will not be able to write to the file. In the text editor, these
DOMExceptions are handled in the saveFile()
method.

What else is possible?

Beyond reading and writing files, the Native File System API provides
several other new capabilities.

Open a directory and enumerate its contents

To enumerate all files in a directory, call chooseFileSystemEntries()
with the type option set to 'openDirectory'. The user selects a directory
in a picker, after which a FileSystemDirectoryHandle
is returned, which lets you enumerate and access the directory’s files.

Opening a file or saving a new file

File picker used to open an existing file for reading.

When opening a file, the user provides permission to read a file or
directory via the file picker. The open file picker can only be shown via
a user gesture when served from a secure context. If
the user changes their mind, they can cancel the selection in the file
picker and the site does not get access to anything. This is the same
behavior as that of the <input type="file"> element.

File picker used to save a file to disk.

Similarly, when a web app wants to save a new file, the browser will show
the save file picker, allowing the user to specify the name and location
of the new file. Since they are saving a new file to the device (versus
overwriting an existing file), the file picker grants the app permission
to write to the file.

Restricted folders

To help protect users and their data, the browser may limit the user’s
ability to save to certain folders, for example, core operating system
folders like Windows, the macOS Library folders, etc. When this happens,
the browser will show a modal prompt and ask the user to choose a
different folder.

Modifying an existing file or directory

A web app cannot modify a file on disk without getting explicit permission
from the user.

Permission prompt

Prompt shown to users before the browser is granted write
permission on an existing file.

If a person wants to save changes to a file that they previously granted
read access, the browser will show a modal permission prompt, requesting
permission for the site to write changes to disk. The permission request
can only be triggered by a user gesture, for example, clicking a “Save”
button.

Alternatively, a web app that edits multiple files, like an IDE, can
also ask for permission to save changes at the time of opening.

If the user chooses Cancel, and does not grant write access, the web
app cannot save changes to the local file. It should provide an alternative
method to allow the user to save their data, for example providing a way to
“download” the file, saving data to the cloud, etc.

Transparency

Omnibox icon indicating the user has granted the website permission to
save to a local file.

Once a user has granted permission to a web app to save a local file,
Chrome will show an icon in the omnibox. Clicking on the omnibox icon
opens a popover showing the list of files the user has given access to.
The user can easily revoke that access if they choose.

Permission persistence

The web app can continue to save changes to the file without prompting as
long as the tab is open. Once a tab is closed, the site loses all access.
The next time the user uses the web app, they will be re-prompted for access
to the files. In the next few milestones, installed Progressive Web Apps
(only) will also be able to save the handle to IndexedDB and persist access
to handles across page reloads. In this case, an icon will be shown in the
omnibox as long as the app has write access to local files.

Feedback

We want to hear about your experiences with the Native File System API.

Tell us about the API design

Is there something about the API that doesn’t work like you expected? Or
are there missing methods or properties that you need to implement your
idea? Have a question or comment on the security model?

Problem with the implementation?

Did you find a bug with Chrome's implementation? Or is the implementation
different from the spec?

File a bug at https://new.crbug.com. Be sure to include as
much detail as you can, simple instructions for reproducing, and set
Components to Blink>Storage>FileSystem. Glitch
works great for sharing quick and easy repros.

Planning to use the API?

Planning to use the Native File System API on your site? Your public support
helps us to prioritize features, and shows other browser vendors how
critical it is to support them.